Monday, March 26, 2012

Summary: Over the past few weeks I found two bugs in unordered-containers. I strongly recommend everyone upgrades to unordered-containers-0.2.1.0 or later, which fixes these bugs.

Both Shake and Uniplate use the unordered-containers library, which provides fast Map and Set types. Both my libraries had bugs reported against them which turned out to be introduced by unordered-containers-0.2.0.0. As soon as these bugs were identified Johan released a new version (0.2.1.0) fixing the bugs, and both my packages now work as expected.

Bug 1: Shake breaks because delete fails

Just over two weeks ago Evan Laforge reported he'd started seeing "thread blocked indefinitely in an MVar operation" if there was a compile error. Shake goes to a lot of effort to clean up nicely, so something was going wrong. Since Evan's project compiles on Linux, and I only have access to Windows machines, I tried replicating it on one of my projects. I perturbed the project in several ways and did manage to replicate the error once, but the same perturbation next time made it succeed - something was clearly non-deterministic (which is the norm in a highly concurrent program such as Shake).

I'd been wanting to write a random build system generator and tester for a while, to thoroughly test Shake, and used this bug as motivation. I wrote the tester, and usually within 5 random runs (about 60 seconds of churning) the error was reproduced on my desktop. However, the error wouldn't reproduce on my laptop even after an hour (because I was on an older version of unordered-containers, I now know), and as I was off travelling I had to put the bug away to look into on my return.

On my return I investigated further. Figuring out the command line to run the random test took about an hour (note to self - write these things down). Once I had that, it still reproduced reliably in about 60 seconds. I cut out the random parts of the test that I didn't think were contributing to the bug (tests for building successfully, minimal rebuilding etc) and cut the reproduction time down to about 5 seconds. I then started augmenting the code with print statements. The cleanup code for exceptions is almost exclusively within the thread pool implementation (see Development.Shake.Pool if you are reading the source). In particular, the code that cleans up after an exception is:

This code finds all threads in the thread pool, with the exception of this thread (the Set.delete t), and kills them. After that has finished, signal to the thread who first called the thread pool that everything has finished with an exception.

Adding trace statements at every step showed that the exception started being cleaned up, a handful of threads were killed, but not all the threads were killed and the barrier was never signaled. An initial guess was that my thread was being killed, and thus that Set.delete had failed to remove t. Since I had debugged a separate unordered-containers bug a week before (bug 2 started after and finished before this one), I went to the unordered-containers commit list and immediately found a bug fixed against delete. I upgraded, and ran my random tester for 3 hours without failure.

Bug 2: Uniplate breaks because filter fails

The Uniplate bug was reported just over a week ago, while I was travelling, by Aleksey Khudyakov. This bug report included a relatively small test case demonstrating a clear bug, along with the vital piece of information that the test case worked with unordered-containers-0.1, but not 0.2. With that information the question became whether Uniplate was using some non-guaranteed behaviour (such as assuming the result from Set.toList is ordered), or whether unordered-containers had a bug. The supplied test case was very sensitive - it involved three modules, and simply changing any module name caused the bug to disappear. The code was also exercising a very sensitive and complex part of Uniplate. After 12 hours of cooperative debugging between myself, Aleksey and Johan, I came up with the reduced test case:

This printed False/True, when it should print True/True. Of course, the choice of the good and bad lists is critical, and was based on the Hashable instance for TypeRep, which uses the fully qualified type name, and thus changes if you rename the modules.

Once I had provided Johan with the reduced test case, it was fixed the same day.

Sunday, March 04, 2012

Summary: I've just released the NSIS library on Hackage. It's useful for building Windows installers, but is also a call-by-name embedded two-stage programming language.

I've just released a new library on Hackage, named NSIS. It's a library for building Windows installers (see an example at the top of the documentation), based on the NSIS (Nullsoft Scriptable Install System). The basic idea of NSIS is that you write a program which defines your installer - it can search for files, create registry keys, create groups of files etc. But, at its heart, it is a full programming language, merely optimised to the job of writing installers. The original NSIS system has an excellent backend, producing small but featureful installers which run well on a variety of Windows platforms. Unfortunately, the front-end programming language is probably best described as assembly code with gentle influences from scripting languages. My NSIS library defines an embedded Haskell language, of the style promoted by Lennart Augustsson, which can produce scripts to be fed into the original NSIS compiler.

Why might you want to use the original NSIS backend?

I've tried several installer generators over the past few years, and even written my own from scratch. Of all these, I prefer NSIS mainly for two reasons:

It's a full programming language. The installer is capable of expressing any program, which means that as you need to do custom things in the installer, you can. For example, you can prohibit installing into the Windows directory. Equally, you can calculate the 1000th prime number. You are never artificially limited by the installer.

The installers are slick and robust. As the NSIS documentation says, "Being a user's first experience with your product, a stable and reliable installer is an important component of successful software". NSIS consistently delivers a smooth end-user experience, provided you select the MUI2 (Modern User Interface 2) settings.

Why might you want to use my frontend?

There are many advantages to my frontend. If your script is simple it is likely to look relatively similar in either system. If your script is complex it could end up 100 lines shorter and far clearer in my system. However, there are three primary advantages.

MUI2 by default

If you are writing a simple NSIS installer, the biggest difference is that my system will use the MUI2 settings by default. As NSIS has evolved there are now three separate ways to define your user interface, using the Classic builtin commands, using MUI1 or using MUI2. Of these, MUI2 is by far the nicest and should always be used, but the Classic interface is built in by default, and MUI2 is defined on top using macros. To specify that you want to insert a particular page into the installer you can write either of:

page Directory # classic NSIS!insertmacro MUI_PAGE_DIRECTORY # MUI2

However, with my front end, you simply write:

page Directory

Flow control

While NSIS installer scripts are very powerful, they aren't easy for script authors. Taking the example from earlier, let's warn before installing into the Windows directory or the System directory:

The original NSIS system has global mutable variables, 16 register variables and a stack - it directly mirrors assembly programming. In my system, variables are properly scoped and named. The difference for complicated functions is significant. As an example, let us define substring replacement in the original NSIS script:

The difference is immense - the first is hard to follow without plenty of paper. The second is a fairly obvious imperative algorithm for string replacement (and is included in the package for reuse). Note that even though we're using a functional host language, our embedded language is very much imperative.

What technologies went into the frontend?

I really enjoyed writing this installer frontend. I got to dust off many techniques from compiler design that I haven't used for a while. Below is a flavour of some of the techniques I used:

Phantom types - all expressions have a phantom type, indicating what type the expression returns. In truth, all NSIS types are strings, but the phantom types let us make conversions explicit and catch user errors.

Call-by-name - the programming language I've implemented is call-by-name, which seems to be a particularly convenient choice for embedded languages.

Optimisation - since the target is a programming language, I wrote eight optimisation passes, which I iterate over. The result is that the string replacement function above becomes 21 lines of compiled code, while the original hand-coded version is 32 lines. There's no real need for the optimisation pass (the code is rarely a bottleneck or a significant size cost), but writing optimisers is fun.

Generics - the front end and optimisation passes are heavily based on generic programming, in particular using Uniplate.

Two-stage programming - the first program runs and generates a second program that is then rerun. It is possible to do computation at either level, and most errors are only caught at one or other of those levels, not both.

Goto programming - while I provide concepts such as iff and while, the target language is exclusively goto based, which I translate down to.

Overloaded literals - I use the overloaded strings and numbers extensively. Haskell doesn't permit overloaded booleans, but if it did I'd use those too.

If any of these techniques are particularly interesting, please comment below, and I'll expand on that area in a future post.